27 Jun 2023
 | 27 Jun 2023
Status: this preprint is currently under review for the journal HESS.

Electrical conductivity fluctuations as a tracer to determine time-dependent transport characteristics in hyporheic sediments

Jonas Leon Schaper, Olaf A. Cirpka, Joerg Lewandowski, and Christiane Zarfl

Abstract. Assessing water transport in riverbed sediments is important for quantifying the effective reactivity of hyporheic sediments and the magnitude of groundwater-surface water exchange flows. A typical approach of estimating transport in riverbed sediments is by measuring natural tracers such as heat or electrical conductivity (EC) and fitting models to them that assume time-independent travel time distributions, implying steady-state flow. Here, we use a transport parameterization that is based on the advection-dispersion equation (ADE) with coefficients that continuously vary in time. The ADE is solved numerically and its solution is fitted to measured EC time series using Bayesian parameter inference. A continuous function of model parameters is constructed by smoothly interpolating between point values with different temporal resolution, and Tikhonov regularization is used to avoid spurious parameter fluctuations. The approach is tested using EC time series synchronously measured in surface water and hyporheic porewater of two urban rivers in Germany and one urban river in South Australia. For all datasets the goodness of fit was improved by introducing a time-dependent EC offset. Estimated porewater velocities were highly transient in three out of the four datasets with values increasing by a factor of 6 over the course of 24 h and were likely related to both variations of hydraulic gradients along and spatial shifting of flow paths. Non-parametric deconvolution indicated that transport in three out of four datasets could be characterized as Fickian, but that flux transients may induce multimodality in stationary travel time distributions. Given the high temporal dynamics, transport characteristics encountered in the streambed sediments of the three investigated urban rivers, we envision that the presented model is a valuable tool to improve the accuracy of both reactive transport simulations and assessments of biogeochemical turnover in riverbed sediments.

Jonas Leon Schaper et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2023-141', Anonymous Referee #1, 28 Aug 2023
  • RC2: 'Comment on hess-2023-141', Anonymous Referee #2, 09 Sep 2023
  • RC3: 'Comment on hess-2023-141', Anonymous Referee #3, 25 Sep 2023

Jonas Leon Schaper et al.

Jonas Leon Schaper et al.


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Short summary
In this study, we present a model approach to quantify river water to riverbed sediment travel times as a continuous function of time using natural electrical conductivity fluctuations as a tracer. We show that apparent water travel times from surface waters through riverbed sediments can be highly dynamic, which may be caused by actual variations of porewater velocity following diurnal variations of head gradients or by a shift of the spatial arrangement of flow paths and their lengths.